The invention relates to an analog-to-digital (A/D) converter and is specifically directed to means for trimming the device in the form of a single chip integrated circuit (IC) in the semiconductor wafer fabrication process. The invention makes use of a Double Digital-to-Analog Converter (D-DAC) disclosed and claimed in our copending application Ser. No. 968,329 filed Dec. 11, 1978, (now U.S. Pat. No. 4,198,622) which is a continuation-in-part of application Ser. No. 879,648 filed Feb. 2, 1978 (now abandoned). These converters make use of a Precision Plural Input Voltage Amplifier and Comparator disclosed and claimed in our copending application Ser. No. 872,966 filed Jan. 27, 1978 (now U.S. Pat. No. 4,191,900). The teaching in these prior applications is incorporated herein by reference.
The prior applications disclose how a simple pair of digital-to-analog (D/A) converters can be combined to create a larger converter with the aid of a precision comparator. Thus two 3-bit D/A converters can be made to produce a 6-bit device with a great saving in parts count. It is further shown how four 3-bit D/A converters can be connected using a plural input comparator to create a 12-bit device with an even greater parts count saving.
The following chart shows the character of various D/A or A/D converters. It is assumed that a 5 volt device is to be characterized. Resolution is expressed in the number of steps associated with the converter. The error column is in % associated with .+-.1/2 LSB. The last column shows the size of 1/2 LSB is terms of voltage.
______________________________________ BITS RESOLUTION ERROR (%) 1/2 LSB (mv) ______________________________________ 6 64 0.8 39 8 256 0.2 10 10 1024 0.05 2.4 12 4096 0.01 0.6 14 16384 0.003 0.15 ______________________________________
It can be seen that even an 8-bit device must be constructed to better than 0.2% overall tolerance if its full capability is to be available. This sort of tolerance is at best difficult to achieve in production. Clearly, a 3-bit device would be relatively easy to build and four 3-bit devices can be built on an IC chip so they closely match. Thus, it is only necessary to trim three of the four devices so that all four match. As shown in our U.S. Pat. No. 4,198,622, a 12-bit A/D converter can be fabricated on a single IC chip. However, to achieve a 12-bit accuracy in mass production some sort of trimming would be necessary. For example, a laser can be used, as is well-known in the art, to trim resistors. Alternatively, the capacitors in the plural input comparator could be laser trimmed for balancing the two D-DAC's. One form of capacitor trimming is disclosed and claimed in copending application of Thomas P. Redfern titled TRIM STRUCTURE FOR INTEGRATED CAPACITORS Ser. No. 877,915 filed Feb. 15, 1978 (now U.S. Pat. No. 4,190,854).
Resistor trimming can be accomplished to a high degree of accuracy. For example, a film resistor can be made to have a lower than desired value and a laser beam or abrasive blast used to remove a portion of the film, thus raising its value. The trim can be done while monitoring resistance to achieve close tolerance. However, it has been found that such trimmed resistors may drift after trimming and such drifting can be accelerated by thermal cycling. This makes long-term accuracy difficult to achieve. It is far more desirable to employ digital trimming where an element is either present or absent. For example, fusible links can be used to join a group of resistors in a series parallel combination, the total value of which can be varied by selective fuse link blowing. While this results in a step wise change of parameter, once the step is achieved it will not drift as a result of trimming. Furthermore, by a careful design of the network, a useful range of trim and precision can be achieved. Also, numerous equivalents of fusible links are available in the art.